The boys and their parents will of course find out who reported them, and it will, I promise, stop this from happening again. The parents will have a very strong incentive to crack down hard on their kids, because if they don't, they could get in trouble with the law, both civilly and criminally. Their sons could wind up being prosecuted in the juvenile justice system. And above all, your victim nephew will form the impression that when bad things like this happen to him, society cares, and acts decisively to stamp it out.